1. Field of the Invention
The present invention relates to a system for controlling and monitoring a gas burner, and particularly to such a burner which is used for heating glass-ceramic cooking surfaces.
2. The Prior Art
Gas burners and radiant gas burners for heating glass-ceramic cooking surfaces typically have a closed burner space with an exhaust gas channel. The flame has a fixed adjustment, so that the heat energy supplied by the burner per unit period of time is constant. The time average heat output of the burner is controlled by a time-pulsed basis, so that the average rate of heat energy produced in any period of time is the integral of the pulses over that time. Ignition of the gas flame and extinguishing thereof are controlled automatically, with the aid of thermostats or the like. An ignition safety device monitors the burner flame, in order to shut off the gas supply when the flame is extinguished. It also blocks the gas supply if the gas fails to ignite from an ignition spark, or when the ignition spark is defective.
Glass-ceramic cooking surfaces of the type referred to, which are heated with radiant gas burners, pose particular problems with respect to the control of the gas in response to various parameters. For example, the average rate of energy, and burner temperature must be regulated in dependence on the thermal load of the cooking surface, which varies with different sizes of pots, and with pots having different heat conductivities and heat absorption characteristics. It is essential that overheating of the underside of the cooking surface be avoided by recognizing a tendency toward an overheated condition in time to avoid it by controlling a temperature-limiting device.
In the general field of gas burners, ignition safety devices, and devices for effecting a maximum temperature limitation, typically take the form of separate devices for each separate task. In addition, yet another separate device is generally employed for regulating the average energy output of the unit. Some ignition safety anf flame monitoring devices employ photoelectric cells are ionization sensors for checking the flame. Other devices operate with a bi-metallic element which operates in conjunction with a pilot flame. In still other cases, gas valves may be directly driven by thermosensitive elements. Typically, the maximum temperature control function is performed by rod expansion switches or by bi-metal switches. The average energy output of the burner is typically controlled by bi-metal switches or use liquid expansion switches or gas expansion switches, and with apparatus for operating the burner on a time-pulsed basis, independently of the burning chamber temperature.
When the conventional sensing elements are employed with burning chambers of radiant gas burners, the installation of such elements causes great difficulties because of the relatively small space available within the burning chambers. Bi-metallic thermosensitive elements cannot be used for energy control or for limiting the maximum temperature of radiant gas burners, because the temperature of the burning chamber, which is approximately 900.degree. C., is too high for such elements. Moreover, the use of rod expansion-type switches for controlling the maximum temperature is unsatisfactory, since the rod expansion switch can react only to the average temperature of the burning chamber, and cannot respond to localized overheatings of the glass-ceramic cooking surface. Accordingly, when rod expansion switches are used, the maximum temperature of the burning chamber must be limited to a relatively low temperature level, and the burner cannot be used to its full capabilities without danger of localized overheating.
Also, control of the average energy output of a burner by means of liquid expansion or gas expansion switches can be performed only indirectly, since these devices are limited to operating temperatures of below 300.degree. C. An indirectly operable liquid expansion switch is disclosed in German Patent Application P 26 21 801.9, which employs a heat accumulation chamber which is separate from the burning chamber, but which is connected thereto. A disadvantage of this kind of control is the fact that the provision of the accumulation chamber causes a reduction in the effective size of the burner, so the heat output of the burner is reduced. Other disadvantages of indirect liquid expansion switches relate to the thermal inertia of the accumulation chamber, and the criticality of the adjustment of physical parts of such devices. The same is true for the gas expansion-type sensors. Other types of known energy control devices, and ignition safety devices, are also accommodated only with difficulty in the small burning chamber. Also, the devices used heretofore typically require a relatively large amount of external wiring, which it is desirable to avoid.